Tyrosine production (an index of the overall rate of cellular proteolysis) was measured in slices of 1) normal canine myocardium, 2) 1-hour ischemic endocardium, 3) 3-hour ischemic epicardium, and 4) 3-hour ischemic endocardium each incubated in "aerobic" or "ischemic" medium. Tyrosine production of normal myocadium in "aerobic" medium was greater than that of ischemic tissues in "ischemic" medium, and was similar to that of ischemic tissues in "aerobic" medium. Thus, cellular proteolysis appears decreased during acute myocardial ischemia, suggesting that cellular proteases do not contribute to ischemic cell death.